Mobile vibrators are often used for producing seismic vibrations. The vibrators are usually attached to a vehicle which is constructed specifically for this purpose.
In a vibrator, instead of pulse excitation, a succession of vibrations of relatively small pressure amplitude and lasting several seconds is produced. The vibration frequency can be varied according to selected time functions. Non-linear frequency vibrations of virtually any type can also be achieved with the aid of suitable control apparatus.
A vibrator has the main components, a base plate and a reaction mass, which together can be caused to vibrate relative to each other by means of a piston-cylinder unit. The entire vibrator is connected to the vehicle by a lifting and lowering device. To use the vibrator, the base plate is lowered onto the ground and pressed against the ground by means of the lifting and lowering device. The pressing continues to the extent that the vehicle is raised at the front or the rear such that substantially the entire weight of the vehicle rests on the vibrator as a static preload. Relatively large amplitude vibrations can thus be achieved.
With the use of such vibrators, P-waves are usually produced. A desired directional characteristic can be achieved by synchronous, dephased excitation of a group of vibrators arranged spaced apart. Thus, it is also possible to produce shear waves with such vibrators.
German patent document A 2,509,704 discloses a vehicle having a vibrator in which the vibrator is arranged at the free end of a frame-type support which is linked to a chassis and can be swiveled relative thereto. A lifting device is attached with spacing from the ground vibrator between the chassis and the swivelable support. In this apparatus, the lifting device includes a piston-cylinder unit whose ends are pivotally connected to the chassis and to the swivelable support frame. The swivelable support frame has swivel arms which are arranged on the chassis and to whose ends the support plate of the vibrator is attached. Arranged parallel to the swivel arms are compensating arms which, due to their telescoping construction, allow the base plate of the vibrator to be tilted ion the longitudinal direction of the vehicle.
Although this arrangement has proven to be expedient over many years, its design is very costly and is no longer in tune with current design possibilities.
German A 1,473,922 discloses a vibrator which is attached to a vehicle and can be raised and lowered by two lifting cylinders arranged at the side of the vehicle. The connection part between the base plate and the lifting part is of flexible construction to compensate for unevenness of the ground. However, this has the disadvantage that the transmittable vibration energy is impaired and the flexible part is additionally subjected to great wear.
U.S. Pat. No. 3,866,709 discloses a vibrator which is attached at the upper end to a holding bar having a spherical end. This also allows the vibrator to be tilted our of the horizontal plane. However, this arrangement does not specify any possibility of imposing the entire weight of the vehicle on the vibrator.
German A 2,737,991 shows a seismic wide-band vibrator whose base plate can be tilted out of the horizontal plane by a special arrangement of stabilizing bars. However, the vibration system and the reaction mass remain in a vertical alignment relative to the vehicle such that a shearing effect can occur due to the angular arrangement of the base plate relative to the reaction mass, which shearing effect can lead to rapid wear of the vibrator and undesirable generation of shear waves.
German A 2,745,213 discloses a vehicle having a seismic source in accordance with the impact principle wherein a central cylinder unit suspended by universal joints serves for raising and pressing the base plate against the ground.
If the force applied is insufficient, additional piston-cylinder units can act directly on the base plates through fork connections. However, the direct action of these units has the disadvantage that the vibrations produced are transmitted to their full extent to the vehicle.